Silica glass optical fibers have revolutionized data transmission, sensing and laser development over the past 50 years. Moreover, dielectric waveguides with a hollow core offer exciting development possibilities beyond traditional technology. Hollow Core Optical Fibers (HCFs) have been fabricated over the past 20 years with various geometries and refinements, yet their attenuation has remained significantly higher than can be routinely achieved in standard silica single mode fibers. Here we present recent developments in Nested Anti-resonant Nodeless Fiber (NANF) design over the last few years and show how this rapidly developing technology has been refined to produce state of the art HCFs at wavelengths between 850 – 1625 nm.

Multi-moded, anti-resonant hollow-core fibre shows great promise for a range of applications from high power laser delivery to novel, non-linear experiments. Anti-resonant fibers typically guide multiple transverse modes due to their large core size, albeit with often large differential modal loss. Understanding the behaviour of higher order modes in these fibres is of crucial importance if we are to exploit the benefits of hollow-core optical guidance in few- and multi-moded applications, or to design more robustly single mode fibres. In this work we conduct thorough numerical investigations into the origins of confinement loss in tubular anti-resonant fibres and its dependence on the fibre's key structural parameters. We show that away from the resonances in the glass, leakage loss can be interpreted as originating from resonant out-coupling between the core modes of interest and the lossy modes of the cladding tubes, and is highest when these are phase-matched. Using this insight, we show that the loss can be estimated a posteriori from knowledge of the fields of the core-guided and cladding tube modes and their propagation constants. Such a quantitative estimate is satisfactory by considering only the three lowest mode groups of the cladding tube. This deeper understanding paves the way to a more informed approach to designing few and multi-moded hollow-core fibers for various applications.

We investigate the design of hollow-core fibers for the delivery of 10s of kilowatt average power from multi-mode laser sources. For such lasers, delivery through solid-core fibers is typically limited by nonlinear optical effects to 10s of meters of distance. Techniques are presented here for the design of multi-mode anti-resonant fibers that can efficiently couple and transmit light from these lasers. By numerical simulation we analyze the performance of two anti-resonant fibers targeting continuous-wave lasers with M2 up to 13 and find they are capable of delivering MW-level power over several kilometers with low leakage loss, and at bend radii as small as 35 cm. Pulsed lasers are also investigated and numerical simulations indicate that optimized fibers could in principle deliver nanosecond pulses with greater than 100 mJ pulse energy over distances up to 1 km. This would be orders of magnitude higher power and longer distances than in typical machining applications using the best available solid core fibers.

Hollow-core fibres typically guide a single low-loss, degenerate spatial mode. Here we present techniques to design HCFs guiding multiple modes with low loss that could be employed in short-haul datacoms or in power delivery. © 2021 The Author(s)

Using hollow-core NANFs with 5-nested-tubes, we achieve the lowest loss ever reported in a hollow core fiber at 1300 and 1625nm (0.22dB/km), and in any type of optical fiber at 850nm (0.6dB/km) and 1060nm (0.3dB/km).

We present antiresonant hollow-core optical fibre designs for VCSEL-based short-reach transmission applications in the 850nm band. Our simulations show that lower loss and twice the bandwidth of solid, multi-mode, graded index fibres are possible.